In optical transmission systems optical signals are transmitted over an optical fiber. Different kinds of modulation and associated types of modulators are used to generate appropriate transmission signals. A common modulator is the QAM (Quadrature Amplitude Modulation) or IQ modulator (I—in-phase, Q—quadrature). A common structure of such an IQ modulator is a nested MZM (Mach-Zehnder Modulator), which consist of two combined parallel working Mach-Zehnder modulators. Optical IQ modulators, independent of their respective realization, have to be adjusted internally in order to deliver the wanted optical waveform. Misaligned modulators insert unwanted spurious by-signals into the optical waveform which distort the optical transmission. The adjustment has to be online, i.e. during operation of the device in order to avoid traffic interruption. The adjustment is either continuously or it takes place every few seconds to minutes. Other problems which have to be solved in case of very high requirements on signal quality is to minimize the effects of the non perfect transfer functions of the analog modulator drivers and a not perfect balance, optical power and electro-optic transfer function within the IQ modulator.
The basic arrangement of an IQ modulator is shown in FIG. 1.
State of the art to adjust the bias of MZM1 and MZM2 is to apply a pilot tone with a frequency f3 on the phase shifters P1, P2 and to apply a further pilot tone with a frequency of f4 on P3, P4, and with help of a monitor diode to minimize the output signals at the modulation frequencies f3 and f4 respectively (and/or maximize the output signals at double modulation frequencies 2*f3 and 2*f4; the pilot tones with frequencies f3 and f4 are shown later in the embodiments of the invention).
The more sophisticated task is to set the 90° point between output components IOS and QOS of the main MZM to achieve e.g. ideal quadrature phase-shift modulation or single sideband modulation.
An IQ modulator and a simple biasing method is e.g. described by Kaoru Higuma et al. in IEICE 2006, Electronics Express Vo. 3; No. 11, pages 234-242, DOI: 10.1587/elex. 3.238. The IQ modulator comprises only one phase shifter electrode per MZM to control the transfer function and to control the phase difference between the signals output from the parallel MZMs This multifunctional modulator can work as a single sideband modulator, a QPSK modulator and so on. It is necessary to control the modulator automatically to avoid unstable operation.
The patent US 2007/0133918 A1, which is included by reference, describes also an IQ modulator with two parallel MZMs wherein only one phase shifter electrode per MZM is necessary to control the transfer function and another phase shifter electrode is necessary to control the phase difference between the output signals from the parallel MZMs.
The patent also discloses a method and system for automatic feedback control of the optical IQ modulator for generating of a quaternary phase-shift-keyed signal without pilot tones. The method comprises the steps of detecting a part of an output signal from the QPSK modulator and processing the signal in the frequency domain to optimise the output data quality by automatic feedback control. The method comprises minimizing (or maximizing) output signals with the help of a gradient algorithm by applying a dithering voltage to an allocated phase shifter. A phase shift of ΔΦ=π/2 is controlled by adjusting a minimum RF output power.
The method described above for use with IQ modulation like (D)-QPSK is easy to implement but has some severe disadvantages with respect to single sideband modulation. If the IQ modulator is used for single side-band generation with many carriers which may be individually switched on or off, this prevents an algorithm who adjusts the 90° setting in the main MZM with help of minimizing the RF output power.
Another disadvantage of the method is the need for partial derivatives this implies low noise monitor signals and the absence of electrical disturbances by imperfect electrical analog and digital circuitry. Additionally a broadband monitor diode is needed for power detection within a several GHz wide band.